A radio-frequency device is one which operates in an environment wherein a characteristic impedance or desired transfer impedance may be identified or specified. Such device might be a discrete transistor in an amplifier circuit, such as for example a field-effect transistor or an integrated unit providing rf amplifier functionality or contributing to such functionality. Devices may operate for example at very high, ultra-high microwave or gigahertz band frequencies. Typical applications would be in the communication area, such as for example transmitter amplifiers, receiver front ends and intermediate frequency stages. Circuits for providing such functionality may for example be based upon FETs, e.g. Gallium Arsenic devices or bipolar transistors, such as heterojunction bipolar transistors.
A package for an rf device is required to provide protection to the device, for example environmental protection for a semiconductor providing an integrated circuit. Equally, a package may provide a degree of integration for example of a semiconductor die or dies and wire components into a single device realizing an amplifier. Such a package also provides terminals for electrical connections to the device itself so that circuits incorporating the device may be realized. Typically, conductive pins are provided which extend beyond the package to be connected into a circuit, for example by soldering to printed circuit board. Within the package terminal extensions which lead towards the device are eventually terminated proximate the device, the final connection being effected by, for example gold wire bonds between bond pads of a semiconductor device and the interior termination sites of the terminal extensions.
A technique for providing such terminals that is adapted for automated production involves providing terminals and their extension as fingers on a carrier portion which is eventually discarded, the whole being known in the art as a lead frame. In production the lead frame is held proximate a semiconductor die, for example so that gold wire bonds may be effected. Therebetween the die and the finger portion of the lead frame is then encapsulated to form a packaged device. The carrier portion of the lead frame may then be separated leaving the connected finger portion extending from package interior to exterior so forming the designed terminals. Use of lead frames has become widely adopted inter alia because it will be apparent that the lead frame prior to separation of the carrier portion may be used to assist work piece support and handling during production.
Many devices generate considerable heat in use and it is well known that such heat must be sunk away to preserve device operating characteristics, device reliability and to prevent damage. To this end, it is known to incorporate a heatsink member within a package for a device. Typically a heatsink might comprise a heat conductive and receptive member being exposed within a package and in thermal contact with a device such that it may be radiated by and/or conducted or convected away from the heatsink. Such heatsink might also be in thermal contact with device terminals or terminal extensions. In some arrangements, it may be appropriate to provide a path or fixing to a terminal or terminal extension, particularly in the case of a lead frame such a fixture may be beneficial since, in addition to providing for example, electrical contact (such as in the case of an rf design requiring a proximate ground plane), thermal contact (such as to aid heat sinking) may be provided. Typically, a fixing would comprise a rivet, for example a rivet formed by protrusion of a rivet shank through a heatsink and a lead frame finger with subsequent formation of a rivet head. It will be appreciated that such fixings enhance lead frame support during production.
When a device package is considered perhaps the most noticeable feature is the encapsulant, since it provides the bulk of externally visible structure. Encapsulants have been developed which can be applied at very low cost and are used to package many commercially available ics such transistor-transistor logic, micro-processors and dynamic or static random access memories. Since many packaged devices are non-rf devices wherein the electrical properties of the encapsulant show little or considerably reduced likelihood of impairing device electrical characteristics appreciably, a diversity of low cost encapsulants and packages have been developed. It will be appreciated that packaging/device cost is the predominant motivator of such development, a package often being commercially accepted with no more than minimal electrical properties.
In rf applications, however, poor electro-magnetic performance can seldom be tolerated and indeed to this end comparatively expensive packaging techniques are presently in use. Ceramic packages (good di-electric properties) and even shielded (metal clad device) packages are presently in use in rf applications. Unfortunately, the expense associated with such techniques implies costs which might limit the progress of otherwise viable technologies.
For example, viability of public cordless telephony might be considered to depend upon availability of a low cost rf amplifier, such that a generally affordable telephone transmitter/receiver handset might be realized. These considerations go beyond low cost packaging considerations, however since a cordless environment is unable viably to tolerate high inefficiency in spite of low cost due to, for example, limited energy storage capacity (e.g. battery life) and limited heatsinking capability (i.e. portable equipment).
It is against such background that the present invention has been made, although it will be apparent that the scope of the invention extends significantly therebeyond.